U.S. patent application number 09/994419 was filed with the patent office on 2002-06-27 for optical power limiting control.
Invention is credited to Patel, Jayantilal S., Zhuang, Zhizhong.
Application Number | 20020080462 09/994419 |
Document ID | / |
Family ID | 22977763 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020080462 |
Kind Code |
A1 |
Patel, Jayantilal S. ; et
al. |
June 27, 2002 |
Optical power limiting control
Abstract
A controllable phase plate has numerous domains that are
randomized as to the orientation of their birefringence and can be
used in a power limiting control to produces an electrically
controllable diffraction pattern having a portion, especially the
zero mode axial spot of the pattern, that is directed onto an
output aperture such as a pinhole or an optical fiber end.
Controlling the phase plate produces an interference peak or null
(or an intermediate level) of light, coupled into the output
aperture. The phase plate preferably comprises a liquid crystal
with controllable birefringence. The domains have paired orthogonal
orientations, which is a condition that is met in randomized
domains. The paired orthogonal orientations make the device
polarization insensitive. In a controllable attenuating device,
collimating lenses are placed before and after the phase plate
along a beam path to focus a clear interference pattern on a screen
containing the output aperture. Several variations are disclosed
including an electrically controllable phase plate arrangement
using liquid crystal controllably birefringent material prepared in
a polarization insensitive manner in zones, or preferably by
providing random director orientation in a plane.
Inventors: |
Patel, Jayantilal S.;
(Newtown, PA) ; Zhuang, Zhizhong; (Yardley,
PA) |
Correspondence
Address: |
DUANE MORRIS, LLP
ATTN: WILLIAM H. MURRAY
ONE LIBERTY PLACE
1650 MARKET STREET
PHILADELPHIA
PA
19103-7396
US
|
Family ID: |
22977763 |
Appl. No.: |
09/994419 |
Filed: |
November 27, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60257792 |
Dec 22, 2000 |
|
|
|
Current U.S.
Class: |
359/256 ;
359/238 |
Current CPC
Class: |
G02F 1/21 20130101; G02B
6/266 20130101; G02F 2203/48 20130101 |
Class at
Publication: |
359/256 ;
359/238 |
International
Class: |
G02F 001/03; G02F
001/01; G02B 026/00; G02F 001/07 |
Claims
We claim:
1. An optical device, comprising: an optical element to be placed
along a propagation path of light, the optical element defining a
plurality of individual domains forming local parts of an area of
the optical element such that at least more than two of the domains
intersect part of the path of the light; wherein each one of said
domains has optical properties that can be different along two
mutually perpendicular axes, said two axes for each said domain
defining a given orientation of said domain; wherein the given
orientations for said domains vary across the area of the optical
element such that any first one of said domains can be paired with
a second one of said domains wherein the optical properties of the
second one of said domains are substantially equal to the first one
but are orthogonal to the first one.
2. The optical device of claim 1, wherein the optical properties
for the two mutually perpendicular axes in each said one of said
domains are substantially equal for at least one of the two
mutually perpendicular axes for all said domains.
3. The optical device of claim 1, wherein said first one and second
one of the domains that can be paired have substantially a same
size.
4. The optical device of claim 1, wherein the optical element
comprises a birefringent material subdivided into said domains such
that the given orientations are defined by orthogonal fast and slow
axes of birefringence.
5. The optical device of claim 4, wherein the retardations across
said domains are controllable.
6. The optical device of claim 4, wherein the birefringence across
said domains is substantially equal
7. The optical device of claim 1, wherein the domains are randomly
sized and randomly oriented across the area.
8. The optical device of claim 4, wherein the birefringent material
comprises a liquid crystal material, and the liquid crystal
material has at least two different director orientations at right
angles to one another and at a controllable angle to a light
transmission axis.
9. The optical device of claim 8, wherein the liquid crystal
material is nematic.
10. The optical control device of claim 8, wherein the controllable
angle with respect to the light transmission axis, is substantially
zero in absence of a perturbing field and non-zero in presence of
the field.
11. The optical device of claim 4, further comprising an optical
system that allows light to pass through the optical element at
least once, such that light from individual said domains interferes
to produce an interference pattern.
12. The optical device of claim 11, further comprising at least one
aperture wherein the interference pattern is created such that more
or less light energy is directed to the aperture by controlling
optical properties of the domains.
13. The optical device of claim 11, further comprising an optical
system that allows the light to pass through the optical element
and to reflect back through the optical element at least once,
whereby the device has a reflective mode wherein an input and
output may occur on a same side of the element.
14. The optical device of claim 11, wherein the optical system
comprises a fiber lens collimator.
15. The optical device of claim 5, wherein the retardation of all
the domains in the element is controllable together.
16. The optical device of claim 1, wherein the optical element
defines an array of said local parts, each having a discrete area
containing said domains.
17. An optical control device, comprising: a structure defining a
propagation path with an input and an output for an incident light
beam, the propagation path having at least a portion in which the
light beam is directed along a light transmission axis; wherein the
output is at least partly defined by at least one aperture, the
aperture having a limited size in a direction perpendicular to the
light transmission axis, whereby light aligned to the aperture can
be passed through the control device, whereas light misaligned to
the aperture is at least partly blocked; a phase interference
element placed along the propagation path, the phase interference
device producing an interference pattern over an area that is
larger than the aperture; wherein the phase interference element is
controllable to vary the interference pattern such that more or
less of the light energy is aligned to the aperture, thereby
controlling an extent of coupling between the input for the beam
and the output.
18. The optical control device of claim 17, wherein the phase
interference element comprises an electrically controllable phase
plate.
19. The optical control device of claim 18, wherein the phase
interference element comprises a birefringent element having a
controllable optical retardation.
20. The optical control device of claim 19, wherein the phase
interference element comprises randomly oriented domains of
birefringent material, wherein a magnitude of retardation of said
domains is alterable by application of an external stimulus.
21. The optical control device of claim 19, wherein the phase
interference element comprises a liquid crystal material, and the
liquid crystal material has at least two different director
orientations at right angles to one another and at a controllable
angle to the light transmission axis.
22. The optical control device of claim 20, wherein the
controllable angle with respect to the light transmission axis, is
substantially zero in absence of the stimulus and non-zero in
presence of the stimulus.
23. The optical control device of claim 21, wherein the liquid
crystal material has multiple domains, and the director orientation
is random for said domains.
24. The optical control device of claim 17, wherein the
interference pattern has a zero order position that is aligned to
the aperture, and the phase interference element varies a
proportion of light coupled to the aperture versus another
proportion of light that is blocked at the aperture, by providing
one of a peak and a null at the zero order position.
25. The optical control device of claim 17, wherein at least one of
the input and the output contain a coupling for an optical
waveguide.
26. The optical control device of claim 17, wherein at least one of
the input and the output contain a coupling for an optical fiber
waveguide.
27. The optical device of claim 20, wherein the magnitude of said
retardation is altered by application of an electric field
simultaneously to a plurality of said domains.
28. The optical device of claim 20, wherein the domains have a size
selected for optimal performance at least at one predetermined
wavelength.
29. The optical device of claim 28, wherein the predetermined
wavelength is approximately 1550 nm.
30. The optical device of claim 20, wherein the magnitude of said
retardation is substantially equal over the plurality of
domains.
31. A method of controlling light transmission along a propagation
path between an input and an output for an incident light beam that
is directed along a light transmission axis, comprising the steps
of: providing an output structure defining at least one aperture
placed for passing light to the output, in a material that
otherwise blocks light from reaching the output; providing a
controllable phase transmission element along the transmission path
leading to the output; directing the incident light along the
transmission axis toward the output, through the phase transmission
element, so as to produce an interference pattern containing peaks
and nulls over an area that is larger than the aperture;
controlling the phase interference element to vary the interference
pattern such that more or less of the light energy is aligned to
one or more of the peaks and nulls that corresponds to the
aperture, thereby controlling an extent of coupling between the
input for the beam and the output.
32. The method of claim 31, wherein said phase interference element
comprises a material having a plurality of birefringent domains
with controllable birefringence, and further comprising orienting
directors of the domains in at least two orthogonal directions so
that the interference pattern is varied for both of two orthogonal
polarization components in a polarization insensitive manner.
33. The method of claim 31, wherein the phase interference element
comprises a liquid crystal with substantially no birefringence
absent a perturbing electric field, due to homeotropic
corresponding alignment of directors at least at one surface of the
liquid crystal cell, and wherein said controlling of the phase
interference element comprises applying an external stimulus
controllably to induce birefringence in said phase interference
element.
34. The method of claim 33, wherein the external stimulus is an
electric field applied across a plurality of domains in the phase
interference element having randomly oriented directors.
35. The method of claim 33, wherein the homeotropic alignment is
obtained at least partly by domains having oriented directors.
36. The method of claim 31, wherein the phase interference element
has substantially non-zero birefringence absent a perturbing
stimulus, due to homogeneous alignment of directors in domains, and
further comprising application of a stimulus controllably to
decrease a value of said birefringence in randomly oriented domains
in the element.
37. The method of claim 36, comprising selecting a degree of
randomness by partially orienting directors in a homogeneous
alignment layer.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority of U.S. provisional
application Ser. No. 60/257,792, filed Dec. 22, 2000.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention concerns devices and methods for controlling
the relative power level of electromagnetic energy propagating
along a path, and in particular concerns use of electrically
controllable interference effects in one or more liquid crystal
elements, to vary the extent to which incident light energy
directed along a transmission path is allowed to pass through an
output orifice, thus being transmitted through the device, versus
light energy directed along other paths, thus being blocked by the
device.
[0004] The invention is particularly useful in pixel displays,
fiber optic data transmission lines and other applications that
advantageously either switch incident light on and off or
controllably attenuate the light. The device operates without
polarizers and at low voltages.
[0005] The device relies on a particular physical layout whereby a
controllable element, preferably a liquid crystal, is placed to
intercept light oriented to impinge on an output orifice. In the
preferred embodiment the controllable element comprises an
electrically controllable random phase plate which creates phase
interference in the forward direction, such that the amount of
phase cancellation along a zero order path to the output can be
electrically controlled.
[0006] 2. Prior Art
[0007] It may be advantageous for various reasons to control
whether or not light energy will be transmitted or blocked at a
given point. In connection with displays, for example, it is useful
to individually control transmission of light at each point or
pixel in an array. In connection with light transmission devices
such as glass fiber optical waveguides and the like, it may be
desirable to switch light on and off for signaling or perhaps to
control the transmission amplitude or light intensity over a
control range. It is particularly advantageous if these sorts of
controls can be effected electrically, that is by application of a
voltage or current signal to alter light transmission conditions.
Other control parameters besides electrical inputs may also be
useful, such as mechanical or temperature related parameters.
[0008] One technique for electrical control of the properties of
light is to pass the light through a liquid crystal. It is known to
provide an array of liquid crystal elements defining pixels in a
display device. Liquid crystals are also employed as elements in
certain types of filters, e.g., in optical transmission situations
such as glass fiber optical waveguides.
[0009] Liquid crystals produce or rely upon light polarization
effects because they have distinct optical properties in mutually
perpendicular axes; i.e., they are "birefringent." The two axes are
known as the "fast" axis and the "slow" axis, or sometimes as the
"extraordinary" and the "ordinary" axes, n.sub.e and n.sub.o. The
liquid crystal material is typically positioned relative to an
incident light beam such that these two axes, n.sub.e and n.sub.o,
define a plane normal to the propagation direction of incident
light (the "z axis").
[0010] The incident light also has distinct spatial components,
namely components aligned wholly or partly to one or both of two
mutually perpendicular polarization axes. There is an interaction
between the polarization attributes of the incident light and the
birefringence axes of the liquid crystal material.
[0011] This interaction is further affected by applying an electric
field in the z axis direction (or potentially by using other
effects such as temperature variation). Assuming an electro-optic
effect, applying an electric field along the z-axis can alter the
birefringence of the liquid crystal, specifically by changing the
index of refraction of the liquid crystal material along the fast
axis n.sub.e, and not along the slow axis n.sub.o. As a result, the
polarization component of the incident light that corresponds to
n.sub.e may experience different optical changes compared to the
component corresponding to n.sub.o. In short, the polarization
components can be affected differently by passing through the
liquid crystal. A polarization filter or a beam splitter responsive
to polarization may then be used to discriminate between the
respective components, for example to turn on or off a pixel in a
display or otherwise to operate light as a function of
polarization.
[0012] Such polarization and birefringence aspects may be useful
but not all of their characteristics are necessarily advantageous.
For example, assuming randomly polarized incident light, a device
with a polarization dependent transmission aspect inherently
rejects 50% of the incident light energy. For this reason,
electro-optic liquid crystal birefringence effects may be
inconsistent with the need to preserve available light energy so as
to maximize the brightness of a display. In some situations it may
be possible to employ polarization diversity techniques to preserve
the light energy. This could involve serially positioned components
to split, realign and recombine orthogonal polarization components
to reduce or eliminate rejection as a function of polarization.
Such techniques entail expense, bulk and potential light energy
losses reasons other than polarization rejection, such as
elongation of the beam path.
[0013] Liquid crystal material conventionally is oriented to a
reference direction on a substrate. In some processes this involves
rubbing or abrading a surface of a substrate. At least for some
thickness, molecules that are spaced inwardly from the surface tend
to align with the elongations of abrasion, known as the "rubbing
axis."
[0014] Typically display devices that use discrete liquid crystals
to control pixel brightness rely on polarization effects. For
common polarizer based displays, the backlighting must be polarized
so that switched effects relying on polarization achieve reasonably
good contrast. This results in at least a 50% loss of possible
light energy. In most polarizer-based displays, two polarizers are
involved. One polarizes the incident light and another
discriminates on the basis of polarization aspects that are
switched on or off at each pixel. This has led to efforts to
develop single polarizer based devices or perhaps dual orthogonal
discrimination elements. Ultimately, it would be advantageous to
eliminate polarizers.
[0015] One device that does not use polarization is the polymer
dispersed liquid crystal (PDLC). This device operates on a
principle of scattering the light when in an "off" state and
passing the light (i.e., becoming transparent) in the "on" state.
One disadvantage of a polymer dispersed liquid crystal (PDLC)
device is that a polymer matrix surrounds the liquid crystal. The
polymer matrix becomes part of an effective voltage divider, and
reduces the voltage applied across the liquid crystal. The
proportionate voltage reduction is determined by the effective
capacitance of the polymer versus that of the liquid crystal. In
some situations, to compensate for a considerable voltage drop
across the polymer matrix, relatively large voltages must be
applied across the device, e.g., on the order of 100V.
[0016] The switching operating principle of the PDLC is
electrically to cause or prevent a mismatch in the index of
refraction between the matrix and the liquid crystal. This changes
the transmissivity/reflectivity characteristic of the boundary,
making the light/dark appearance of the pixel controllable
electrically.
[0017] It would be advantageous if a light handling technique could
be developed that was free of the light energy rejection
inefficiencies associated with polarization. However it would also
be advantageous if the technique used low control voltages and
modest power dissipation as typical of the electro-optic
birefringent liquid crystal displays. Preferably, such a technique
would reject as little incident light as possible, at least
preserving more than the 50% level typical of a simple polarization
dependent display. The technique should achieve a very high degree
of contrast, using a low voltage, a good response time, and do so
with a minimum requirement of additional components.
SUMMARY OF THE INVENTION
[0018] It is an object of the invention, among other objects, to
employ the birefringence aspects of a controllable liquid crystal
element to produce switchable phase interference effects that
control transmission of light along a transmission path, in a
manner that is insensitive to polarization.
[0019] It is also an object to remove reliance on polarization in a
way that does not require preliminary steps to separate orthogonal
polarization components, and to align and recombine them, so as to
preserve the amplitude of the incident light energy.
[0020] It is another object to achieve very high contrast between
switched and unswitched conditions, using modest control voltages,
while also permitting a continuous range of control when
desired.
[0021] It is an object to optimize a device that meets all the
foregoing objects, for applications including high density pixel
displays in the visual band on one hand, and also fast switching
glass fiber optical waveguide applications in the 1550 nm
range.
[0022] These and other objects are accomplished by a controllable
phase plate that produces a diffraction pattern having a portion,
especially the zero order mode axial spot of the pattern, that is
directed onto an output aperture such as a pinhole or an optical
fiber end. By controlling the phase plate, an interference peak or
null (or an intermediate level) is coupled into the output
aperture. The phase plate preferably has a liquid crystal with
controllable birefringence in small domains that have orthogonal
director orientations. In a preferred arrangement the directors are
randomized. This makes the device polarization insensitive. The
device relies on having a propagation path for a light beam
directed toward the output aperture, with the controllable phase
interspersed along the path. Preferably, collimating lenses before
and after the phase plate along the path produce a clear
interference pattern focused in the area of the output aperture.
Several variations are disclosed including an electrically
controllable phase plate arrangement using liquid crystal
controllably birefringent material prepared in a polarization
insensitive manner in zones, or preferably by providing random
director orientation in a plane.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The drawings illustrate certain embodiments of the invention
as presently preferred. It should be understood that the invention
is subject to certain variations from the illustrated embodiments,
within the scope of the invention defined by the appended claims,
particularly insofar as the concepts taught in this disclosure are
applied to practical devices. In the drawings,
[0024] FIGS. 1a, 1b and 1c are schematic illustrations of optical
arrangements according to the invention, wherein light from a
"point" source is collimated and recombined at an output point, the
figures respectively showing a single point source, a linear array
and a pixilated X-Y array of points.
[0025] FIG. 2 is a schematic illustration of an optical control
device according to the invention, shown for one point only.
[0026] FIGS. 3a and 3b are schematic illustrations, respectively
showing unperturbed and perturbed alignments of molecules in a
liquid crystal or similar birefringent material that is subject to
perturbation by application of an electric field in an orthogonal
or Z direction compared to the X-Y plane shown.
[0027] FIG. 4 is a perspective schematic corresponding to a
plurality of zones as shown in FIGS. 3a and 3b.
[0028] FIG. 5 is a schematic illustration of an optical element in
front elevation (the central view) and in two corresponding side
elevations (the lateral views) from opposite sides, showing the
relative alignment of birefringence structures, such as liquid
crystal molecules, according to an inventive aspect.
[0029] FIG. 6 is a three-view composite as in FIG. 5, wherein two
distinct zones are provided, of different relative alignment.
[0030] FIG. 7 is a three-view composite as in FIGS. 5 and 6,
showing an alternative zone arrangement comprising zones having
molecules with different mutually perpendicular orientations.
[0031] FIG. 8 is a schematic illustration of a linear array of
domains having orthogonally oriented directors of two types that
are arranged in alternating order.
[0032] FIG. 9 is a schematic illustration wherein pairs of
orthogonally oriented directors are randomly oriented and arranged
in random order in a linear array of equally sized domains.
[0033] FIG. 10 is a schematic illustration corresponding to FIG. 9,
wherein the domains are of random size.
[0034] FIGS. 11a, 11b, 11c are microphotographs showing a phase
mask element according to the invention, wherein a control voltage
is not applied (FIG. 11a), applied at part scale (11b) and applied
at higher amplitude (11c).
[0035] FIGS. 12a and 12b are section views showing optical power
limiters according to the invention, in transmissive and reflective
embodiments, respectively.
[0036] FIG. 13 is a graphic plot of output intensity versus control
voltage according to an exemplary embodiment of the invention.
[0037] FIG. 14 is a graphic plot of output intensity versus control
voltage under varying polarization conditions.
[0038] FIG. 15 is a graph of output intensity versus time,
including an inset having enlarged scale, showing exemplary minimal
drift in output level over time.
[0039] FIG. 16 is a perspective graph of output intensity versus
control voltage over a range of wavelengths.
[0040] FIG. 17 is a perspective graph showing theoretical
expectations for the range of data shown in FIG. 16.
[0041] FIG. 18 is a graph showing theoretical attenuation level
versus wavelength according to an embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0042] In discussing the illustrated embodiments, this description
uses various terms that denote orientations, relative positions and
the like, such as "vertical," "horizontal," etc. These terms and
others like them are intended to facilitate discussion of the
drawings and not to limit the invention to particular orientations
unless so stated.
[0043] According to the invention, a very high contrast light
modulator is provided, which can function without polarizers, at
much lower control voltages than PDLC light modulators.
Controllable liquid crystal elements are provided with molecules
oriented in a randomized way that produces an optical interference
pattern. The liquid crystals permit the pattern to be altered
controllably to direct light into an output orifice to permit
transmission, or outside of the orifice to block transmission,
depending on the signal applied to the liquid crystal. Likewise the
invention permits controlled transmission of any desired proportion
of the incident light energy between maximum transmission and
blockage. The invention is preferably optimized for operating at
the 1550 nm wavelength commonly used in optical communications.
However the invention is also applicable to other wavelengths such
as the visible wavelengths used in displays. The invention is based
on controllable phase interference, and thus uses a fundamentally
different principle from that of PDLC devices.
[0044] Two structural aspects are needed to provide a controllable
transmission device according to the invention. First, a light path
is provided that is physically arranged normally to allow incident
or input light to pass through because the incident light is
directed toward an exit port. That is, the device has entrance and
exit ports that are aligned. Second, an electrically controllable
random phase plate is placed along the path between the entrance
and the exit. The random phase plate is electrically controllable
so as to adjust conditions of phase interference in a forward
direction from the entrance to the exit. The phase cancellation
conditions along the path from the entrance to the exit port, and
the resulting spatially distributed peaks and nulls, can be
electrically controlled. Thus by controllable phase cancellation,
the device can align or misalign the greater part of the incident
light energy relative to the exit port, thereby transmitting or
blocking light transmission through the device, respectively.
[0045] The exit port can be a confined and masked orifice area
immediately adjacent to a center axis of a light transmission path
from the entrance to the exit. The phase cancellation conditions
preferably are arranged by providing a collimated light beam that
is incident on an electrically controllable liquid crystal element.
The controllable phase cancellation at the exit port pursuant to
the invention, facilitates transmission by providing an
interference pattern in which the light energy is concentrated at
the exit port, substantially because the interference pattern has
an axial or centrally placed peak at the exit orifice. Conversely,
transmission is blocked by producing an axial or central null at
the exit orifice. A liquid crystal controllable element as
described herein thus can be used selectively to concentrate the
incident light energy along a transmission path through the device,
or not, depending on the signal applied to the liquid crystal
element.
[0046] The invention is subject to embodiment in a variety of ways
that produce the desired phase cancellation effect described. An
exemplary embodiment is shown in the drawings, wherein phase
cancellation is produced in a collimated beam.
[0047] Referring to FIG. 1(a), an input source of light energy can
be provided in the form of a point source (A). The source could
also have some finite diameter, but can be appreciated in this
embodiment by considering an ideal point source. The source could
be, for example, a tip of an optical fiber, a focused light spot or
another source. Light from the source is collimated. In the
embodiment shown, the light beam diverges from a point source (A)
and a portion of the light from the source (A) is incident on a
collimating lens (B) placed at a distance from the point source
equal to the focal length of the lens. The light incident on the
lens (B) is thus collimated or redirected into parallel rays
forming a beam of light centered on an axis.
[0048] For purposes of illustration, the lens (B) is assumed to be
a spherical lens of glass or a similar material wherein a
difference in index of refraction redirects the light. It should be
appreciated that various other structures can produce a collimated
beam, including, for example, a parabolic reflector, a Fresnel
lens, a distant diffuse source, etc. (not shown).
[0049] If undisturbed, the beam of collimated light is incident on
a second collimating lens (C), placed along the axis, which can be
the same as the first lens (B), and operates to concentrate the
beam back into a point at the output of the device. The second
collimating lens (C) focuses an image of the input source (A) at
the output. Assuming that the input (A) was a point source, then
this arrangement produces an output image (D) that is likewise a
point. Substantially all the incident light energy in the
collimated beam is focused to point (D). The two lens collimating
system shown produces a replica at the output (D) of a point source
input (A), if there is no blockage introduced along the beam path.
According to the invention, however, phase cancellation is
introduced by placing a controllable liquid crystal along the beam
path.
[0050] FIGS. 1(b) and 1(c) demonstrate a one-dimensional
arrangement and a two-dimensional array of point sources,
respectively. An exemplary one dimensional point source could be a
line of pinholes in a screen backed by a diffuse light source. An
exemplary two dimensional array could be provided by a sheet-like
screen at the entrance with an array of pinholes to act as light
sources. Similarly, the output or light collection point(s) can be
limited to one dimensional or two dimensional arrays of points
formed by pinhole or small diameter openings in an opaque
screen.
[0051] As mentioned above, many other point applications are
possible. For example, for a display application, each point can be
a pixel light source. An array of optical fibers can be bundled and
placed endwise such that the arrayed cores of the fibers act as
light sources. The principles of the invention are described with
reference to single points, but can be applied to a two dimensional
array simply by repeating the individual elements as pixels in an
array of sources, lens elements, liquid crystal phase control
elements and light collection orifices.
[0052] In the simple one-pixel case, such as a single optical fiber
application, point (A) in FIG. 1(a) can be an end of an optical
fiber. The collimating lenses (B) and (C) can be conventional glass
or plastic lenses or gradient index lenses (commonly referred to as
GRIN lenses). Collection point (D) can similarly comprise a fiber
end. In such a system, the light emitted from the fiber at point
(A) is collected by the second fiber at point (D), provided that no
obstruction is introduced in the light path. According to the
present invention, a controllable obstruction is introduced in the
specific form of a liquid crystal element operable to induce a
phase variation resulting in interference.
[0053] The effect of phase variation can be seen in FIG. 2, wherein
a phase mask grating (E) is shown placed in the light path between
the collimating lenses associated with the inventions. If according
to the illustrated structure, the phase mask (E) divides the light
into two components, then interference between the components
produces an interference pattern. The interference pattern produces
an array of peaks and nulls where the waves from the plural
components interfere more or less positively, or cancel one
another. According to the invention, such a pattern is produced
controllably, and is employed to separate the light spatially.
[0054] In the embodiment of FIG. 2, the spaced collimating lenses
(B) and (C) could be at an indefinite spacing if the collimation is
ideal, without changing operation, because the rays of collimated
light would remain parallel. However, the phase mask or grating (E)
preferably is placed in front of the second collimating lens at
exactly at a distance equal to the focal length of the lens. The
light is diffracted by the grating, and a diffraction pattern is
caused by the light propagating forward.
[0055] One component of the interference produces peaks and/or
nulls along the axial beam center line, namely on the zero order.
Progressing divergently outwardly from the phase mask, and
progressively greater angles relative to the center line, are nulls
and peaks at the first, second, third and further orders, etc. The
pattern of this interference forms a diffraction pattern of
divergent rays. The second collimating lens (C) focuses the image
produced by the phase mask onto a screen containing an aperture (D)
for selectively passing only a given order of the diffraction
pattern. Preferably, aperture (D) is an axial aperture and passes
the zero order of the diffraction pattern and blocks the upper
orders, which are focused on the screen containing aperture (D),
but at a radial space out from the center line.
[0056] The zero order of the diffraction pattern produces the
brightest peak, and accordingly produces the best contrast when the
diffraction pattern is varied according to the invention,
controllably shifting the light energy at the zero mode axial
center line between its brightest peak and a substantially complete
null.
[0057] The divergent diffraction pattern emanating from the phase
mask is redirected by the second collimating lens (C) into parallel
rays. As a result, the respective, zero, first, second, etc. orders
of the diffraction pattern are directed into discrete areas of
light then passed through the lens to become parallel beams because
they all emanate from the focus of this lens. Only the zero order
beam passes through the defined by the output aperture (D) and into
the fiber or into another strategically placed destination such as
a hole or sensor. The other light, namely at all the upper orders,
is blocked by failure to align with the aperture (D). In the
embodiment shown, the light signal is attenuated because part of
the light energy is deflected from the zero order into higher
orders. The proportion that is passed along the center line and
focused to a point, versus the portion that is deflected into the
other orders, is controllable according to a further aspect of the
invention.
[0058] Assuming that the phase mask is a grating, the extent to
which the phase mask can discriminate between the respective orders
of diffraction peaks and nulls depends in part on the fineness of
the grating. If the phase grating is relatively coarse, then the
higher order diffraction peaks may pass closely along the center
line of the optical system and thus reduce any contrast between the
zero order and the first order, etc. If on the other hand the
grating is very fine (i.e., has a small period between grating
lines), a higher contrast device is possible.
[0059] According to the present invention, the desired effect can
be achieved with a grating that is not unduly fine. The zero order
spot or brightness peak in the diffraction pattern occurs along the
center axis. The first order spot for such a grating occurs at a
position X.about.f.lambda./d, where f is the focal length of the
lens (C), .lambda. is the wavelength of the light and d is the
distance between lines of the grating. If we arbitrarily choose f=3
mm, .lambda.=1.55 .mu.m, and X=100 .mu.m (which is much larger than
mode diameter of a single mode glass optical fiber) then d=46.5
.mu.m. In other words, the phase grating needs to be about 20
lines/mm or finer, if the zero order and the first order of the
diffraction pattern are to be at least 100 .mu.m apart. Therefore,
the grating does not have to be exceptionally fine to produce the
desired effect.
[0060] In general, the amount of light that is distributed into
various orders of the diffraction pattern can be calculated by
taking the Fourier transform of the pattern. Thus for example if
the pattern is a precisely sinusoidal phase grating, only the zero
and the first order of the diffraction pattern can be expected to
appear. Also, by using this technique, the diffraction patterns
expected from other phase mask patterns can be calculated.
[0061] According to an inventive aspect, a controllable phase mask
was desired. A controllable phase mask was considered using a
liquid crystal material whereby, the phase characteristics are
variable under electric control to change states between one
switchable condition in which incident light was directed along the
zero order of a controllable phase plate, and another switchable
condition in which the incident light was directed elsewhere.
[0062] According to the invention, an electrically controllable
device is provided, which functions as a controllable phase plate.
This is accomplished using the controllable birefringence of a
liquid crystal, rather than by altering the spacing of lines of a
grating as the phase plate.
[0063] The invention preferably uses electro-optic materials, such
as liquid crystals or other materials such as PLZT or lithium
niobate or another material in which the phase retardation of light
passing through the material can be altered in a controlled manner
using external perturbation. The perturbation is discussed herein
using an electric field as an exemplary sort of controlling
parameter. It should be appreciated that there are other possible
perturbing effects associated with thermal, mechanical, magnetic,
optical and other variations besides the preferred electric field
variation.
[0064] In the example of electro-optic effects, liquid crystals are
anisotropic materials in which the phase retardation of light
passing through a device comprising liquid crystal can be varied,
controllably, as a function of the amplitude of an applied electric
field. In a liquid crystal, application of an electric field varies
the index of refraction for plane polarized components aligned to
one of two orthogonal axes. The result of the change in index is a
corresponding variation in the phase retardation or phase delay
that occurs between light that is incident on the device versus
light that passes through the device and emerges.
[0065] Phase retardation occurs when light traverses a medium
having a given length of propagation path. The extent of phase
difference is a function of the material dimensions, the index of
refraction of the material, the wavelength of the incident light,
etc. However, by controllably varying the index of refraction for
one axis, as is possible using liquid crystal material, a
differential phase retardation effect can be produced. The
differential phase can be produced between components of incident
light that align to the controllable axis, versus components that
align to the uncontrolled axis that is orthogonal to the
controllable axis. The differential phase being controlled also can
be on the same axis, for portions of a device that are electrically
controlled versus portions that are not controlled (or even
portions subjected to different control amplitudes).
[0066] The differential phases are arranged to interfere, namely to
be summed as separate sources. Interference peaks occur where the
peaks in the summed phases align in phase. Nulls occur where the
phases are 180 degrees out of phase, because positive peaks of one
sum with negative peaks of the other, resulting in zero
amplitude.
[0067] A simple example of a liquid crystal device for producing an
electrically controlled phase shift according to the invention, is
one based on nematic liquid crystal. An advantage of this material
is that a phase shift sufficient to be used for controllable
attenuation as described can be accomplished with a relatively low
level of applied voltage because such liquid crystal material is
readily deformed by the external perturbation.
[0068] Liquid crystals are birefringent, and the change in index
with applied voltage occurs along a specific polarization
orientation relative to the orientation of molecules of the liquid
crystal. As a result, the electric field does not operate to
produce a phase variation equally on both orthogonal polarization
components. It would not seem to be possible to apply a liquid
crystal in a straightforward manner to produce a useful phase
variation for both polarization axes simultaneously (components
aligned to n.sub.e and n.sub.o of the liquid crystal), and assuming
arbitrary input polarization of the light, except. To usefully
employ the electrically controllable birefringent axis n.sub.e of
the liquid crystal, one would normally expect to need to
specifically design the optical system for polarization
insensitivity, e.g., either to exclude light that is not plane
polarized to the controllable axis, or to divide, reorient and
recombine the polarization components of the incident light to
align to the controllable axis. Such a device is not preferred
because it may involve excluding a large part of the incident light
(the portion that is not polarized to align to the controllable
birefringent axis), or alternatively, separating and reorienting
polarization components in a way that adds complexity and expense
to the optical system.
[0069] The elementary operation of an electrically controllable
liquid crystal can be appreciated by considering plane polarized
light, impinging on a liquid crystal device. A liquid crystal
device can be manufactured, for example, with nematic liquid
crystal, oriented using a surface configuration that forces the
"director" or reference direction be parallel to a given line, such
as the y-axis as shown FIG. 3. It is sometimes said that the slow
axis of the crystal is oriented to a brushing direction.
[0070] The light is to propagate along a z-axis in FIG. 3. The
crystal molecules or elements down through the crystal material,
align with the director absent other influences. Therefore, absent
any perturbation, the molecules or elements have a regular
orientation as shown on the left side of FIG. 3.
[0071] However, by applying an electric field along the z-axis,
which can be done using conductive transparent electrodes that are
deposited on inner surfaces of the cell, the director effectively
can be reoriented. This is generally not uniform. Because of the
rigid boundary conditions at the brushed surface, the molecules
spaced away from the surface, and in the middle of the cell, are
more easily distorted and those at the surfaces are less easily
distorted, which is shown on the right side of FIG. 3. This
produces a non-uniform director distortion across the cell such
that the angle of deviation .theta. away from the x-y plane changes
as a function of z.
[0072] In the geometry being discussed, the tilt .theta.(z) is
confined to the y-z plane. The distortion from the original
orientation depends on the magnitude of the applied electric field
and is shown schematically in FIG. 4. The variation of .theta.(z)
due to the voltage-induced deflection is less at the surfaces and
greater proceeding inwardly away from the surfaces. Thus,
.theta.(z) is different at different points, and the variation is
not linear. The variation and its non-linearity present no
problems, however, because what is important is the total phase
retardation. Deflected molecules or local elements may have a
higher or lower index depending on their position in view of the
variation in deflection proceeding inward from the surface. However
the total phase shift through the crystal is the sum or the
integrated total through all the elements encountered by the light
passing through. Such liquid crystal structures can be arranged in
zones, as shown in FIG. 4, each zone being controlled
independently.
[0073] According to the present invention, a liquid crystal as
described, or a structure having similar attributes, is used in the
position of the phase grating discussed above with respect to FIGS.
2 and 3, to modulate the phase of a source of light contributing to
an interference pattern, such that the pattern is made adjustable.
If the light incident on a liquid crystal in the position of the
phase grating in FIG. 2 has at least a component that is polarized
parallel to the director orientation of the liquid crystal, then
application of an electric field to reorient the director changes
the index of refraction for that polarization component. This
adjusts the phase of that component when emerging from the liquid
crystal and can reposition peaks and nulls of the interference
pattern as a result. That is, for one of two orthogonal
polarization components, it is possible to use a liquid crystal to
modulate the phase of the light traversing the liquid crystal,
specifically for the polarization component that is parallel to the
director axis.
[0074] Thus according to the invention, plural phased light
components are caused controllably to interfere, while effectively
tuning the phase retardation of light through a device used as a
controllable phase grating, and thereby altering the diffraction
pattern produced. Also according to the invention, the diffraction
pattern is normally directed onto a field having one or more
positions where the light or its signal are passed (e.g., by having
an aperture or a glass fiber end or a light sensitive sensor, for
example), versus one or more other positions where the light or its
signal are blocked. By altering the phase retardation of the
controllable phase grating, the invention permits light energy to
be directed to one or another of the positions.
[0075] As discussed above, it is very desirable in any practical
device to be polarization insensitive, preferably in a manner that
does not require splitting diverse polarization components and
recombining them in manner that establishes uniform plane
polarization characteristics.
[0076] FIGS. 5-7 show some simple examples of how such a device can
be embodied by providing zones of liquid crystal material with
different director orientations. Two superposed zones are shown in
the examples, but an example could have any number of zones or
regions having one of two director orientations and positioned in
interleaved order. These structures can produce polarization
insensitive results.
[0077] When birefringent materials are used in a phase mask as
described, and in particular to make a controllable attenuator
according to the invention, the controllable attenuation is
polarization dependent because phase modulation using a
birefringent liquid crystal is specific to one of the two
orthogonal crystal axes. The phase retardation can be tuned for
only one of two orthogonal polarization components of the light.
Nevertheless, the device can be structured so as not to be
polarization dependent, for example by providing areas that operate
equally and concurrently, each handling one of the two orthogonal
polarization components. Some possible orientations of liquid
crystal directors are shown in FIGS. 5-7. In these examples there
are two zones shown in each case. It is likewise possible to use a
larger number of alternating zones instead of only two.
[0078] Operation can be appreciated considering the graphic example
shown in FIG. 8, wherein the arrows at each segment are considered
the director of the local strip or block of liquid crystal
material. First, for example, if the incident light is plane
polarized vertically, then the incident light is aligned by
polarization to the director of one of the two types of block
defining zones. If the incident light is plane polarized
horizontally, then the light is aligned to the director of the
other type of block and the next adjacent zones.
[0079] Both zones are liquid crystal, although in this example they
are interleaved in perpendicularly aligned sets. As discussed, it
is an attribute of liquid crystal that the index of refraction is
electrically controllable on one axis and on the perpendicular axis
the index of refraction is fixed. For any single zone (arbitrarily
named as the "first" zone), a phase difference arises if a voltage
is applied to the liquid crystal, namely a phase difference between
the polarization component along the director, which is subject to
the electrically controllable phase retardation, and the
polarization component that is perpendicular to the director and
thus is subject to a fixed phase retardation regardless of control
voltage.
[0080] In FIG. 8, if all of the zones are subject to the same
control voltage, and the foregoing results accrue for the "first"
zone, then the same results will also occur for the adjacent zones,
except that if the first zone controls the vertical whereas the
horizontal is fixed, then the second zone controls the horizontal
whereas the vertical is fixed. The point is that by representing
both polarization components in both zones, the overall device is
made polarization insensitive. If the first zones have controllable
vertical and fixed horizontal, and the second zones have
controllable horizontal and fixed vertical, and the zones are all
subject to the same control perturbation, then the polarization
components are all handled in the same way.
[0081] Stated in a more mathematical way, the phase difference
between two neighboring zones is d(n.sub.x-n.sub.y)2.pi./.lambda.
for the chosen polarization, and d(n.sub.y-n.sub.x)2.pi./.lambda.
for the orthogonal polarization. If the phase difference obtained
by a given voltage is .phi. for one polarization then (assuming
that the same voltage is applied uniformly across the device) the
phase difference for the orthogonal polarization is -.phi.. If two
parts of light, with a phase difference .phi., are brought together
and interfere with each other, the resulting intensity at zero
order is given by cos.sup.2(.phi./2). This function is insensitive
with respect to the sign of phase difference .phi.. Therefore, the
device is polarization insensitive.
[0082] Any arbitrary polarization of incident light can be
decomposed into field or vector components along perpendicular x
and y axes as shown in FIG. 8. Therefore, the device in FIG. 8 is
polarization insensitive for all input polarizations. Generally
stated, for any arbitrary voltage across the liquid crystal, for
any relative orientation of the incident light to the physical
structure, including the orientations shown in FIGS. 5 through 8
assuming two paired zones, interleaved zones or the like, the phase
difference is simply N and -N for the two orthogonal polarization
components. The devices are insensitive to polarization.
[0083] According to an inventive aspect, the concept discussed as
is applied to an arrangement in which the director is not strictly
arranged in zones but in fact is randomized. This case is
schematically shown FIG. 9. For simplicity, the drawing shows a
series of equally sized domains. Each domain has a director shown
by an arrow, and is construed to contain a controllable
birefringent material such as a liquid crystal, whereby one
orthogonal polarization is subjected to a variable index of
refraction and the other polarization is subjected to a fixed
index. Assuming that for each zone having a given director
orientation, there is another equal zone having a director
orthogonal to the given director orientation, then the device would
be functionally the same as FIG. 8. That is, for sets of
birefringent zones in which there is equal representation of two
orthogonal directions, incident light of any and all arbitrary
polarizations is treated the same. The device is polarization
insensitive.
[0084] For purposes of illustration, the director is shown for each
zone in FIG. 9, and each director is paired by a bracket with
another director and another zone that is orthogonal to the first
director and zone. It is possible to show that if there are a large
number of domains and the directors of the domains are distributed
randomly, then the device is polarization insensitive because it is
possible to pair domains of any director orientation with other
domains of orthogonal orientation. The device can produce a
variable interference pattern as discussed, wherein the zero mode
or axial direction can be varied to become an interference peak or
a null or any attenuation between them, for all polarization
conditions.
[0085] The zones in FIG. 9 are of equal size. If there is a random
distribution of director orientations and also a random
distribution of domain sizes, the same considerations remain
applicable. So long as a statistically large set of domains and
orientations are represented, it will be possible to pair any given
domain by director orientation and size, with another that is equal
bur orthogonal to the first. The result is a tunable diffraction
pattern attenuator structured as shown in FIG. 2, with a
statistically large, number of randomly oriented domains of random
sizes, at least within a range. In short, the device operates as a
polarization insensitive attenuator.
[0086] Thus according to an inventive aspect, the controllable
phase mask (E) used according the invention as shown in FIG. 2, has
a plurality of zones wherein the liquid crystal material is
preferably the same material or otherwise has the same extent of
birefringence, but the directors of the zones are oriented
randomly.
[0087] In the foregoing analysis, one can note that the spacing
between two paired regions with orthogonal director orientations
may be unequal (i.e., the members of one pair may be closer
together than the members of another pair. The spacing between the
phased sources affects the nature of the diffraction pattern that
results from interference between the sources. However, that matter
is of no moment according to a preferred embodiment, because the
aperture at the output device is aligned to the centerline, and
only the zero mode of the diffraction pattern is used to couple or
decouple light energy to the output aperture. The zero mode is the
central, potentially-brightest peak in the diffraction pattern and
is not affected by rotation. However, by varying the phasing of the
numerous randomly oriented zones or domains, by applying a control
voltage commonly to all of the zones or domains, the zero mode in
the interference pattern becomes a peak or a null, or is
controllable to a level between the two.
[0088] Insofar as any two paired orthogonally oriented zones have
non-uniform spacing, the result in the diffraction pattern is that
the higher order spots in the pattern become more blurred. Insofar
as the rotational orientation of the directors of the zones varies,
the diffraction pattern becomes a series of concentric rings.
Nevertheless, the zero order remains controllable and the device
functions substantially to shift a portion of the light energy
between the zero order central spot in the diffraction pattern and
areas spaced outwardly from the center.
[0089] As a result of the foregoing structure, the intensity of the
peak or null at the central zero order spot in the diffraction
pattern is made to be a function of the birefringence of the phase
mask. For liquid crystal or similar materials, the birefringence is
electrically controllable. Therefore, in combination with an
aperture mask that screens off the central zero order spot, the
invention provides an electrically controllable attenuator.
[0090] Analysis shows that the intensity of the zero order spot
scales in a polarization independent manner as a cosine square
function of the birefringence of any one element. The contrast of
the device can be made very high if the birefringence in all
portions or zones of the element is essentially equal. It is also
desirable to minimize variations in thickness while providing a
highly random or at least pseudorandom distribution of director
orientations. If these criteria are not met, then some degradation
of the contrast (difference between highest and lowest light level)
can be expected.
[0091] In illustrating and discussing the invention, a
one-dimensional line of zones or domains was shown for purposes of
illustration. The same considerations apply and can also be
graphically shown or represented by equations concerning an X-Y
array of positions on a two dimensional element having a given
thickness along a center axis.
[0092] According to further aspects of the invention, in order to
provide an electrically controllable attenuator as described, and
in particular to provide a polarization insensitive phase mask, a
controllable phase mask is provided with a substantially random or
pseudorandom distribution of director orientations in zones or
domains over a surface. The domains have an equal value of
birefringence (oriented randomly) that is controllable commonly,
for example by a commonly coupled electrical control voltage.
[0093] According to an inventive aspect, there are two related
versions of randomly oriented liquid crystal domains that permit
polarization insensitive attenuation as discussed above. According
to one approach, the domains have zero birefringence (i.e., equal
indices along their orthogonal axes) in the absence of an applied
electric field, and have increasing birefringence (increasingly
unequal indices) as a function of applied voltage. According to
another approach, the birefringence of the domains is non-zero in
the absence of an applied electric field, and is reduced (the
indices are brought nearer to equal) as a function of applied
control voltage.
[0094] Depending on the material and physical attributes of the
controllable phase mask, it is possible that the output of the
devices as an attenuator may have several peaks and nulls over a
given range of control voltage. This can occur, for example, if the
range of phase retardation produced on the controllable axis of the
phase mask over the given range of control voltage, is greater than
a full period.
[0095] A goal for production of the phase mask is to construct a
liquid crystal structure that produces a high contrast diffraction
pattern, and thus can be used as an efficient light attenuator that
is controllable by application of an electric field. The device
should be uniform as to its phase retardation characteristics
across the surface of the device, both in the absence of an applied
electric field and (at a different phase retardation value) upon
application of the field. Advantageously, the device should be
finely and continuously controllable to establish a desired degree
of diffraction, in a reversible manner without hysteresis. Although
substantial uniformity is desirable as to thickness and
birefringence when perturbed or not perturbed, a high degree of
randomness is desirable as to director orientation.
[0096] Assuming some birefringence in the absence of perturbation,
the attenuator of the invention can be arranged either to minimize
attenuation when perturbed or when not perturbed, and thus to
provide an output spot that is bright or dark accordingly. That is
the device can produce attenuation due to light diffraction in the
absence of the field, and can become clear (minimum attenuation) in
the presence of a field.
[0097] Operation of the attenuator as described has functional
similarities to operation of a polymer dispersed liquid crystal
device (PDLC). However the invention operates at quite low control
voltage by comparison, due to its different operational principles.
A typical operating voltage can vary, for example, by only a few
volts with saturation of electro-optic properties occurring at
voltages below five volts. The attenuator of the invention also
operates in a way that preserves more of the incident light energy
than devices that employ polarization selective steps effectively
discarding one polarization orientation while employing another.
The invention is polarization insensitive and uses both orthogonal
components of arbitrarily polarized input light.
[0098] According to further aspects of the invention, a process is
disclosed for producing a phase mask as described. According to a
preferred example, an electro-optic liquid crystal cell is
provided. It should be understood that although electric
perturbation types are of interest, it is also possible to use
other forms of perturbation such as mechanical variations,
magnetic, thermal, etc. An exemplary phase mask element comprises
indium tin oxide coated conductive glass.
[0099] According to another inventive aspect, the surface
properties of the element are controlled so that the surface
director is randomly oriented at one or both surfaces in the
presence of the perturbing field (preferably an electric field). In
the absence of the electric field, the structure is chosen to be
homeotropic, and in that state the director field is substantially
parallel to the light propagation direction. The liquid crystal
used is one with negative electric anisotropy in one
embodiment.
[0100] Surface preparation is an important part of the process of
producing high contrast, namely a highly diffracting structure in
the presence of an applied field, that also is non-diffracting in
the absence of the field. Alternatively, it is also possible and
may be desirable to produce a structure having the inverse
characteristic, namely being diffracting when unperturbed rather
than vice versa.
[0101] According to one technique, a random structure is produced,
and the surface is coated with a homeotropic surfactant so that in
absence of the field, vertical alignment is obtained. When an
electric field is applied, the molecules tilt towards the surface
to produce a random orientation with substantially the same phase
change along a unique axis of the domain determined by the plane
formed by the tilt direction of that domain. This direction is
randomly oriented for different domains, but the domains have
substantially the same value of the electrically controllable
birefringence.
[0102] An object is to provide a surface that can produce random
orientation when an electric field is applied to the liquid
crystals, but at the same time has the desired properties, allows
deposition and surface bonding of the homeotropic alignment layer,
and has no birefringence except upon application of a control
field. To consider the desired liquid crystal alignment, the
surface topology can be compared to the topography of land. A
randomly non-uniform surface is comparable to a surface with
variations in elevation, perhaps comparable to hilly terrain. This
metaphorical surface of varying elevation carries distributed
standing trees, which are springy and represent vertical structures
corresponding to the directors or orientations of the liquid
crystal molecules. In this analogy, when a perturbing electric
field is applied, the molecules tilt over downwardly and lay in the
valleys, producing a random two-dimensional orientation when
perturbed but not when unperturbed. When the perturbation is
lifted, the trees stand back up again vertical.
[0103] As another analogy one can consider that unperturbed liquid
crystal molecules are aligned with directors like standing blades
of grass on a lawn, presenting their ends to the direction of light
propagation. The perturbing field presses the blades down like an
air current that is more or less strong and pushes the blades over
more or less flat as a result. The blades lay over in random
directions relative to their bases, producing random orientations
of birefringence. The extent of birefringence, like the length of
the grass blades, is substantially equal, regardless of
orientation.
[0104] Consistent with such analogies, the extent of variation in
elevation or the height of the standing blades and the depth of the
surface roughness may be small. However, the birefringence of the
liquid crystal is the essentially of the same magnitude over the
surface, which is different when perturbed versus unperturbed. In
the perturbed state the molecules become randomly oriented as to
their directors or as to their slow and fast axes, n.sub.o and
n.sub.e.
[0105] The goal of producing a random structure was accomplished by
using a polymer with siloxane polymer backbone. A commercial
polymer called GR650F (available from TECHNEGLAS, INC., Perrysburg
Ohio) was used. The choice of this polymer is made only for
illustrative purposes and other similar materials of a type used
for producing conventional liquid crystal products are also
possible. The GR650F polymer produces advantageously produces a
thin clear film when deposited from a solution for example by spin
coating. To provide for surface scattering, very small glass
particulate material (glass powder) was added, which also has the
appropriate surface chemistry to allow the homeotropic agent to
bond to the surface. The powder used in the test example was
Cab-O-Sil (available from Cabot Corporation).
[0106] In particular, a solution of 2% GR650F and 2% L-90
Cab-O-Sil, all by weight and in an ethanol solvent, was mixed to
produce a milky stable solution which was first deposited on indium
tin oxide (ITO) coated glass plates. This coating was dried at
about 120C degrees for 30 min and then subjected to oxygen plasma
etching. This process burned away the organics and produced a
scattering glassy surface on top of the ITO coating.
[0107] This surface was coated with 0.5-5% (by weight) ethanol
solution of Octadecyltrimethoxysilane (available from Aldrich
Chemical Company #37,621-3) or equivalent chemical for inducing
homeotropic characteristics. This was baked at 120C degrees for
about 30 minutes. The plate was then assembled into a liquid
crystal cell by methods commonly known in the art and filled with
negative dielectric anisotropy material. An exemplary such material
used was ZLI 4302 (available from Merck).
[0108] It should be understood that there are other methods that
will be apparent to those skilled in the art whereby such an
optical element with a random scattering surface can be provided,
and which according to the foregoing disclosure has vertical
director alignment in the absence of a perturbing field and in the
presence of the field produces a scattering effect by corresponding
alignment of its structures. For example, another method of making
the desired structure is by mixing 2% GR650F polymer, by 2% of fine
glass powder such as L-90 or Grade PTG or Grade EH-5 and 2%
Octadecyltrimethoxysilane, all by weight and in an ethanol solvent.
This solution is then spun onto a glass plate, for example at about
2000 rpm or by another similar process to make a thin coating. The
coating is dried at about 140C degrees and assembled into cells as
described above and filled with liquid crystal with negative
dielectric anisotropy.
[0109] The inverse structure described above can likewise be
produced by similar means, except that the alignment of the liquid
crystal molecules is arranged to be random in the absence of the
perturbing field and to tilt toward vertical in the presence of the
field when an electric field is applied. In this case a positive
dielectric anisotropy material may be employed.
[0110] As stated above, the designation "vertical" and similar
directional or spatial terms, are used to assist in an
understanding of the invention but are not limiting in an absolute
sense. The term "vertical" as used in the preceding paragraph, for
example, denotes an orientation that is protruding or endwise or
normal (standing) relative to a general plane defined by a surface.
The term "vertical" in this context does not require that the
associated general plane have any particular orientation, whether
horizontal or otherwise, but is used for convenience in explaining
relative orientations.
[0111] A simple example in accordance with the invention is to
produce a homogeneously aligned sample by avoiding conventional
unidirectional rubbing of the sample and similar steps.
Conventional rubbing is considered advantageous in liquid crystals
so as to provide parallel rubbing or abrasion scratches or valleys
that function to align certain of the crystal molecules, causing
other molecules to assume the same alignment. According to the
invention, in the absence of rubbing or other alignment steps,
similar production steps can produce randomly-oriented domain
structures. Alternatively, rubbing or abrasion steps the
specifically produce random orientations in localized areas, can be
employed, such as agitation with a volume of particulate abrasive,
particulate blasting or the like.
[0112] Application of an applied field causes the molecules to tend
to line up with the field, which can increase or decrease the
amount of birefringence, thereby providing a controllable amount of
attenuation using the phase mask and aperture arrangements of the
invention, preferably but not necessarily also including focusing
optics as in FIG. 2. Another example of producing random structure
would be for example, by using the example illustrated above but
not coupling the surface with a homeotropic alignment layer.
[0113] The invention was demonstrated using the procedures outlined
above by first producing a scattering surface and then coating the
surface with a homeotropic alignment agent. The Liquid Crystal ZLI
4302 (available from Merck) was used for the examples that produced
the following results. This material has a negative dielectric
anisotropy.
[0114] FIGS. 11a-c are microphotographs that show the appearance of
the sample when backlighted by placing the sample between cross
polarizers. (There is no collimating or focusing element used for
this depiction, which shows simply how the domains can be
randomized.) The sample is shown as it appears with and without an
applied electric field, viewed as backlighted, and along a line
normal to the planar surface of the sample. In the absence of the
applied field, the sample appears black, which is due to the fact
that the molecules are vertically aligned for the most part, except
for the presence of a few areas in which the molecules are
anomalously tilted. In this geometry, because the light propagates
parallel to the director axis, the sample exhibits no
birefringence. There is no phase interference activity. The sample
appears black, which is to say that little or no light is
transmitted directly through the sample on a line normal to its
plane. When an electric field is applied however, the
formerly-endwise (or "vertically") oriented molecules tilt towards
the plane of the glass, by a degree that varies as a function of
the amplitude of the control voltage. However, the molecules tilt
in random directions. The control voltage causes the material to
become birefringent, with the directors of the molecules being
oriented randomly. The extent of birefringence is tunable with
adjustment of the control voltage.
[0115] Thus the application of the control voltage causes randomly
oriented differential phase retardation, namely between the
polarization components of the incident light that happen to be
incident on randomly oriented molecules that have their fast and
slow axes oriented in the required direction. The incident light
can have any arbitrary polarization mix or can have any specific
plane polarization orientation, which factor is irrelevant to
operation of the invention. When a control voltage is applied to
this embodiment, the controllable phase mask of the invention and
its randomly-oriented birefringent directors, produces differential
phase retardation of polarization components at any and all
orientations. Effectively, the phase mask produces a large number
of paired sources of light energy that are phase retarded by
different amounts.
[0116] The phase retardations fall substantially into two specific
values, namely the phase retardation of the birefringent material
along its fast and slow axes, because all portions of the phase
plate are subjected to equal perturbation (the same control
voltage). The control voltage can vary the difference in phase
retardation by controlling the amount of birefringence.
[0117] The difference in phase retardation produces interference.
Such interference can produce peaks and nulls in various
directions, but by selecting the zero order, namely by directing
the output into an axial or centrally placed aperture, only the
zero order beam is selected and only the selected beam is affected
by the value of the phase difference between two sources. The "two"
sources are the pairs of all the zones with orthogonal orientations
of the director, which are randomly oriented (and thus polarization
insensitive) and are summed by operation of the device as
shown.
[0118] The foregoing physical layout of the device, and the
electrically controllable random phase plate as described, are
useful together to provide an optical attenuator. The invention was
demonstrated with an optical fiber as a point source of light and a
graded index (GRIN) lens as a light collimator. A randomly aligned
nematic liquid crystal sample was used as a means of varying and
controlling the phase as described. As shown in FIGS. 12a and 12b,
this arrangement is applicable to both transmissive and the
reflective geometries for an optical attenuator.
[0119] The optical system used to obtain the experimental results,
which follow, were obtained by using a commercially obtained dual
collimator from CASIX, Inc. (Fujian, China), which allows
convenient use of the reflective geometry. The use of the dual
fiber collimator is advantageous because it requires only one lens
to make an optical attenuator. The random phase plate was made as
described above except with a reflective gold electrode on one side
for a reflective geometry. The collimated light passes through the
sample, is reflected by the gold mirror electrode, and passes back
through the sample and the collimator to be collected by another
fiber as shown in FIG. 12b.
[0120] The optical attenuation performance measured at a 1550 nm
wavelength for a 3.0 micrometer sample is shown graphically in FIG.
13. These measurements were taken using a duel fiber collimator.
The output light amplitude was measured using an optical power
meter. The applied electric field was switched, using a 1 kHz
square wave. For the plotted results shown, the zero dB level
corresponds to the reflected light without the sample in place.
[0121] The results in FIG. 13 show that it is possible to attenuate
incident light by as much as -22 dB using the electro-optic
controllable birefringent element described. The results also show
that the extent of attenuation (and the output light level), is
smoothly controllable by adjusting the amplitude of the electric
field.
[0122] FIG. 13 actually contains two sets of data. The solid line
(which is substantially obscured by the dots) represents light
measurements when continuously changing the applied voltage in step
increases from the previous value. The dots represent the light
measurements when randomly selecting and applying different a
voltage value and measuring the resulting level of attenuation. The
rather precise overlap of these two sets of data shows that there
is no substantial hysteresis in the response, which is another
advantageous aspect of the invention.
[0123] Additional tests and measurements were undertaken to
determine and confirm the polarization independence of the device,
and are shown graphically in FIG. 14. The polarization independence
of the structure was verified using light whose polarization was
varied as a function of time so that incident light with different
polarization states was passed (or not passed) by operation of the
sample.
[0124] A random polarization generator was provided, capable of
generating all states of polarization in a steady state or a
changing random sequence. The results of these measurements are
shown in superimposed plots of dots and squares. The plots overlap
precisely, showing that the device had no detectable polarization
dependence.
[0125] The invention was further tested to assess long term
stability, the results being shown in FIG. 15 over a test period of
about an hour. This test consisted of attenuating the signal to a
predetermined value and monitoring the variation in transmitted
light level as a function of time. Minimal drift was observed,
estimated at about 0.2 dB, which is comparable to the variability
of the input power level.
[0126] FIG. 16 demonstrates that the attenuator of the invention is
wavelength dependent to an extent, which should be expected because
the attenuator essentially relies on producing an adjustable
interference pattern by setting a particular differential phase
retardation. These are wavelength dependent parameters. However for
an operating window of a typical device, such as a 40 nm band
centered at 1550 nm, the wavelength dependence is manageably small,
and can be estimated by considering that the phase change in the
operating range is expected to change by about 2.5%. To measure
this effect, the optical system described with respect to FIG. 12b
was used with a broadband light source to determine the wavelength
characteristics of optical attenuation. Different wavelength scans
and different control voltages were applied to the attenuator, with
the experimental results shown in FIG. 16.
[0127] A wavelength dependent oscillation was observed for the
data, which is believed to have resulted from the fact that no
anti-reflective (AR) coating was applied to the front surface of
the device. To confirm that this was the case, simulations were
run, the results being shown in FIG. 17. The simulations were based
on the known thickness of the glass and the theoretical
mathematical model discussed above, which indicated that the
expected attenuation should be a cosine square function of the
birefringence. The simulation and the observed results were
similar, as shown, which suggests that application of an AR coating
on the front surface should produce results more nearly as shown in
FIG. 17. FIG. 18 shows the expected low wavelength dependence if AR
coating is used.
[0128] The invention has been explained with respect to a number of
specific examples. With the benefit of this disclosure, it should
be apparent that there are variations from the disclosed examples
that are also operative. Such variations are likewise a part of
this invention. Reference should be made to the appended claims
rather than the discussion of specific examples, to determine the
scope of the invention in which exclusive rights are claimed.
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